B1gmode softening in FeF2

Experiments are reported for the temperature dependence of phonon Raman scattering in Fe${\mathrm{F}}_2$. The $B_{1g}$ phonon exhibits anomalous frequency softening with decreasing temperature for temperatures in the range $T_N=78-340\mathrm{}$ K. This softening is less marked for temperatures below $T_N$ because of spin-lattice effects resulting from the antiferromagnetic ordering. In the higher-temperature region the square of the $B_{1g}$-phonon frequency is proportional to temperature, which is suggestive of a virtual structural phase transition. However, consideration of other work on the pressure dependences of the Raman-active modes in similar compounds leads us to conclude that the mode softening results largely from the lattice thermal contraction. This contraction disproportionally influences the forces between nearest-neighbor fluorine ions in adjacent planes perpendicular to the $c$ axis.